Modern well drilling techniques, particularly those concerned with the drilling of oil and gas wells, involve the use of several different measurement and telemetry systems to provide petrophysical data and data regarding drilling mechanics during the drilling process. Data is acquired by sensors located in the drill string near the bit and either stored in downhole memory or transmitted to the surface using MWD telemetry devices. Prior art discloses the use of a downhole device incorporating resistivity, gravity and magnetic measurements on a rotating drillstring. A downhole processor uses the gravity and magnetic data to determine the orientation of the drill string and, using measurements from the resistivity device, makes measurements of formation resistivity at time intervals selected to give measurements spaced around the borehole. These data are compressed and transmitted uphole by a mud pulse telemetry system. The depth of the resistivity sensor is computed at the surface and the data are decompressed to give a resistivity image of the face of the borehole wall with an azimuthal resolution of 30.degree..
Prior art methods are limited to making resistivity measurements in the subsurface and fail to address the issue of other useful measurements that could be made using an MWD device. Prior art devices are also limited to measurement devices that rotate with the drill string and do not take advantage of current drilling methods wherein a mud motor is used and the drill bit could be rotating at a different speed from the drill string or wherein a non-rotating sleeve may be available on which substantially non-rotating measuring devices could be located. The rate at which measurements are made is selected to be constrained by the data transmission rate of the telemetry system. Prior art systems thus fail to take advantage of the inherently higher capability of measuring devices and the ability to use redundant data to improve the signal-to-noise (S/N) ratio. Prior art also relies on an uphole determination of the depth of the tool, whereas if the determination of the depth of the tool were made downhole intelligent decisions could be made about the amount of data to transmit uphole. The present invention overcomes these inadequacies.